The present invention is directed to a liquid gauging system, in general, and more particularly to an improved method and apparatus for determining ultrasonically the quantity of liquid in a tank.
Ultrasonic liquid gauging systems, like a fuel gauging system for an aircraft, for example, generally include one or more ultrasonic transducers at each fuel tank of the aircraft, generally disposed at the bottom thereof, and one or more target reflectors disposed in the tank at predetermined distances from the ultrasonic transducer. In operation, an incipient ultrasonic burst signal is transmitted from the transducer, conducted through the liquid, reflected from the height of the liquid, i.e. the liquid/air interface, and returned to the transducer where it is received. A round trip time period from inception to reception of the ultrasonic burst signal is measured to determine the height of the liquid in the tank. In order to determine liquid height the velocity of sound of the liquid is needed. One technique for determining velocity of sound of the liquid is to utilize the time measurements for the ultrasonic burst reflections from the one or more target reflectors in the tank. Since the distance between a target reflector and the transducer is known the velocity of sound may be determined from said distance and the time measurement for the target reflector.
But this presumes that the velocity of sound of the liquid is substantially constant over a large liquid height profile around the target reflector. Unfortunately, this may not always be the case, especially if the liquid in the tank is thermally stratified. Accordingly, having the velocity of sound at one height of the liquid may not be sufficient across the over all height profile of the tank liquid, especially if accuracy of liquid quantity measurement is of paramount importance. Thus, it would be an important improvement to be capable of determining the velocity of sound cumulatively at the height of the liquid in the tank under thermally stratified conditions.
In addition, stratification may also occur due to a separation of different liquids in the tank. For example, reflections which may occur from the stratified liquid levels, may compromise the time measurements of the reflections from the target reflectors. Therefore, a liquid gauging system may also be improved by distinguishing between the different reflections in order to obtain accurate time measurements from the reflections of the target reflectors.
Also, current ultrasonic transducers like that illustrated in cross sectional view in FIG. 3A, for example, include a bottom layer of piezoresonator material which is of a different acoustic impedance than that of the liquid in the tank about the operational frequency passband of the ultrasonic burst or pulse transmitted and received therefrom. Generally a second or top layer of material is disposed between the piezoresonator material and the tank liquid for matching the acoustic impedances of the piezoresonator material and the tank liquid to render an efficient energy transfer. However, this acoustic impedance matching has not always been accurate due primarily to the available material for use as the second layer. Accordingly, an improvement in efficiency of energy transfer can occur if the acoustic impedance matching is made for accurate than currently implemented.
The embodiment of the invention which will be described in a succeeding section ameliorates the aforementioned drawbacks, thus providing a more accurate and improved ultrasonic liquid gauging system.